: All organisms present circadian rhythm in most of their physiological functions, and among them there stand out sleep, motor activity, immune function, the secretion of melatonin, and the production and release of numerous neurotransmitters, in particular of serotonin because of its relationship with the aforementioned factors. Aging changes these rhythms, altering sleep quality and contributing to immunosenescence. Treatment with exogenously administered melatonin or tryptophan may restore these impaired functions due to aging. In our animal model (Streptopelia risoria), both the hormone and the amino acid acted on the activity-rest rhythms, modulating the circulating levels of melatonin and serotonin, and increased the cell viability and resistance to induced oxidative stress of blood heterophils, at the same time as enhancing the phagocytic function and neutralizing the superoxide anions deriving from this immune function. Also, in the old individuals, the treatments with melatonin and tryptophan at the concentrations and times of administration considered suitable improved nocturnal rest besides reverting the immunosuppressory and oxidative effects accompanying phagocytosis at these advanced ages.